Within some Aikido circles, I have heard it be said that the jo-staff was representing a spear thus should be treated as if it had a blade at one end. This help explains uke's reactions to some of tori's attacks in the later sections of the goshin no kata. I am having a hard time reconciling a jo with a spear for obvious reasons.

So, my questions is what is the origin of the jo in Aikido? and as an aside what weapon, if any, is the jo meant to represent?.

re your aside: My opinion is that the jo staff is not meant to represent anything. It is studied for the direct purpose of improving one's Aikido.
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Craig ConstantineAug 13 '13 at 14:17

@CraigConstantine: That opens a can of worms as to why uke does the things that uke does. With a blade at the end, most moves/counters makes sense. If there is no blade, things become a little more muddied. Of course, we both agree that both Aikido jo and Aikido bokken are tools to learn Aikido first and learn some cool wooden weapons skillz second.
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SardathrionAug 13 '13 at 14:39

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It is likely that much of O-sensei's training and inspiration in jō came from his close personal relationship with Nakayama Hakudō (中山博道), who held menkyō in Shintō Musō-ryū (among others). It's important to remember that while Ueshiba Morihei introduced jō into the art, it was largely codified into a specific skill set by Saitō Morihiro, who first published the techniques in 1973. The jō is not representative of anything other than a staff; specifically of a common length of walking stick used by itinerant monks and common travelers.
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stslavikSep 23 '13 at 20:16

3 Answers
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It is borrowed/taken from various other arts (rifle/bayonet, spear(yari), and specific jo arts) with the founder of Aikido then blending/creating his "Aiki jo" art.

Stan Pranin of Aikido Journal (formerly AikiNews) has written:

The exact origins of the Aiki Jo remain somewhat of a mystery. Some
have found traces of Morihei Ueshiba’s jo movements in the “juken” or
rifle with bayonet he practiced as a young soldier. Others point to
the influence in the Aiki Jo of the “yari” or spear that he studied
with intensity during the Ayabe period. It might also be noted that
the Founder was exposed to many classical systems due to his wide
network of associations in martial arts circles

The quote is take from here. But you can find a few other apropos items on Aikido Journal.

From other things I've read from Stan, I think the above quote sums up what little concrete knowledge there is.

From my understanding, and this is an un-sourced comment from my instructor (Aikikai), the jo represents a rifle/bayonet combo. If you consider that O-Sensei developed Aikido during various wars (largely formalized pre-WW2), this makes a lot of sense.

However, I have no sources, so this is faith in my instructor on my part, and pure speculation for the rest of you!

I'm sorry I don't have any sources for this, but I have always heard it that O-sensei really liked the movements of the bo, however, the ceiling of the dojo was (usually?) too low, so he adapted the bo movements into use of the jo (with which you can do hassō indoors).

Except in France, there it is a spear, sometimes with a straight blade at one end and a curved blade at the other. I personally find this explanation much less likely.